1. Field of the Invention
The present invention relates to a solid-state imaging device and to a semiconductor photoelectric conversion device having a photoelectric conversion element such as a photodetector of a photocoupler, and more particularly to a solid-state imaging device that is interchangeable in fabricating processes with a CMOS (Complementary Metal-Oxide Semiconductor) device, i.e., a CMOS image sensor.
2. Description of the Related Art
Recent years have seen the development of cameras that are used for acquiring image data and used together with, for example, personal computers (PCs). Charge coupled device (CCD) image sensors that employ CCDs or CMOS image sensors that are interchangeable with CMOS devices in fabrication processes are used as the solid-state imaging devices that are incorporated in these cameras.
A CCD image sensor is a type of image sensor in which photoelectric conversion elements or photodiodes are arranged two-dimensionally corresponding to pixels (picture elements), the signals of respective pixels that have become electric charges by means of photoelectric conversion elements being read sequentially using vertical transmission CCDs and horizontal transmission CCDs. CMOS image sensors are similar to CCD image sensors in that photoelectric conversion elements are arranged two-dimensionally corresponding to pixels, but in reading signals, rather than using vertical and horizontal transmission CCDs, signals stored for respective pixels are read from selected picture elements by means of selection lines constituted by aluminum lines, as in the read-out of a semiconductor memory device.
In contrast with a CCD image sensor, which requires a plurality of positive and negative power source voltages for driving the CCDs, a CMOS image sensor can be driven by a single power supply and enables lower power consumption and lower power source voltage than a CCD image sensor. Furthermore, the use of a fabrication process for the CCD itself in the fabrication of a CCD image sensor complicates the straightforward application of fabrication processes that are typically used for a CMOS circuit. In contrast, the fabrication processes used for a CMOS image sensor are also commonly used for CMOS circuits. Peripheral circuits such as logic circuits, analog circuits and analog/digital conversion circuits can therefore be formed simultaneously with the CMOS image sensor by means of CMOS fabrication processes that are often used in the fabrication of processors, semiconductor memory devices such as DRAMs (Dynamic Random Access Memories), and logic circuits. In other words, a CMOS image sensor has the advantages that it can easily be formed on the same semiconductor chip as semiconductor memory device or a processor, and in addition, the fabrication of the CMOS image sensor can easily share the same manufacturing plant as a semiconductor memory device or processor.
FIG. 1 is a schematic plan view showing an example of this type of CMOS image sensor and shows the floor plan of a semiconductor device that is formed as a CMOS image sensor. CMOS image sensor 1 is provided with: imaging unit 2 in which photoelectric conversion elements are arranged two-dimensionally for each pixel; timing generator 3 for generating timing signals that are necessary for reading signals from the pixels; vertical scanning unit 4 and horizontal scanning unit 5 for selecting the output of pixels; analog signal processor 6 for amplifying and processing signals from selected pixels; and logic circuit unit 7 for processing the analog signal output from analog signal processor 6 and outputting the result as digital signals. Logic circuit unit 7 is provided with: A/D converter 8 for performing analog-to-digital conversion of the input analog signals; digital signal processor (DSP) 9 for converting the digitized signals to digital image signals; and interface (I/F) 10 for outputting digital image signals to the outside and receiving command data from the outside.
Explanation next regards the unit cells that make up imaging unit 2 of CMOS image sensor 1. The unit cell in this case is provided for each pixel and is constituted by a photoelectric conversion element, i.e., a photodiode, for each pixel realized by a PN junction, and a transistor that constitutes a switch for selecting this photoelectric conversion element. FIG. 2 is a schematic sectional view showing the construction of an unit cell of the prior art in a CMOS image sensor.
Unit cell 11 has fundamentally a construction in which a p-type well region 13 is provided on pxe2x88x92-type substrate 12, and n-type photoelectric conversion region 14 which joins p-type well region 13 to form a photodiode is provided in the surface of p-type well region 13. For the purpose of isolating this unit cell 11 from adjacent unit cells, there is further provided: p+-type isolation region 15 that is formed in p-type well region 13; isolation oxide film 16 formed on, for example, p+-type isolation region 15; gate oxide film 17 which is formed on portions of the surfaces of p-type well region 13 and n-type photoelectric conversion region 14 other than the region in which isolation oxide film 16 is formed; interlayer insulation film 18 which is formed so as to cover the entire surfaces of isolation oxide film 16 and gate oxide film 17; and shield film 19 which is formed in interlayer insulation film 18 for preventing the incidence of light to unnecessary portions.
In addition, n+-type reset drain region 20 is formed in p-type well region 13 at a position that is somewhat separated from n-type photoelectric conversion region 14. Gate oxide film 17 is also formed on the surface of this n+-type reset drain region 20. Reset transistor 21 is formed which takes the region that is within p-type well region 13 and between n-type photoelectric conversion region 14 and n+-type reset drain region 20 as the channel region, n-type photoelectric conversion region 14 as the source region, and n+-type reset drain region 20 as the drain region. N-type photoelectric conversion region 14 is thus connected to n+-type reset drain region 20 by way of reset transistor 21.
Unit cell 11 is further provided with driver transistor 22 of a source follower, and transistor 23, which is a horizontal selection switch. N-type photoelectric conversion region 14 is connected to the gate of driver transistor 22 for outputting to the outside output changes according to the amount of incident light. Load transistor 24 of a source follower is formed for each row of the unit cell array. Driver transistor 22, transistor 23, and load transistor 24 are inserted in that order in a series between power supply voltages VDD and VSS. The voltage output Vout of this unit cell 11 is obtained from the connection point between transistor 23 and load transistor 24.
A CMOS image sensor of this construction operates as follows.
First, raising a pulse which is applied to the gate of reset transistor 21 to a high level sets the potential of n-type photoelectric conversion region 14 to the power supply voltage VDD which is applied to n+-type reset drain region 20 and thus resets the signal charge in n-type photoelectric conversion region 14. Lowering the pulse which is applied to the gate of reset transistor 21 to a low level brings about the start of accumulation of signal charge. During accumulation of signal charge, the incidence of light generates electron-hole pairs in the region of the lower portion of n-type photoelectric conversion region 14, whereupon the electrons are accumulated in the depletion layer below n-type photoelectric conversion region 14 and the holes are discharged through p-type well region 13. The potential of n-type photoelectric conversion region 14 then changes according to the number of accumulated electrons, and by the operation of the source follower, this change in potential is outputted by way of the source of driver transistor 22 to horizontal selection switch transistor 23, whereby a photoelectric conversion output characteristic having good linearity can be obtained.
Although xe2x80x9ckTCxe2x80x9d noise occurs in n-type photoelectric conversion region 14, which becomes a floating diffusion layer, when reset transistor 21 is reset, this noise can be eliminated by sampling and storing output during darkness before transmitting signal electrons and then finding the difference between this and bright output. Here, k is Boltzmann""s constant, T is temperature, and C is electrical capacitance.
FIG. 3 is a schematic sectional view showing the a photodiode structure which provides greater reduction of leak noise than the photoelectric conversion element (photodiode) in the unit cell shown in FIG. 2. In FIG. 3, parts bearing the same reference numerals as parts in FIG. 2 are the same constituent elements as in FIG. 2.
The photodiode structure shown in FIG. 3 includes surface p+-layer 25 formed on the surface of the photodiode (i.e., photoelectric conversion element) portion in FIG. 2, and the provision of this type of surface p+-layer 25 reduces leak current on the surface of the photodiode portion. This type of construction is well used in the prior art.
Recent years have seen increasing demand for miniaturization as well as demand for greater numbers of pixels in image sensors (i.e., solid-state imaging devices), and these demands have inevitably necessitated both higher integration of the unit cells and smaller area per pixel. A reduction in the size of a pixel results in a reduction in the area of each photodiode, and this reduction in area in turn results in a decrease in the amount of light that is incident to each pixel. The reduction in the amount of incident light has resulted in a drop in output signal, whereby the SN (signal-to-noise) ratio of the output image is degraded and image quality deteriorates.
A solution to these problems calls for an improvement in the photoelectric conversion efficiency per unit area. Since the pixels are smaller, the amount of light that is incident to each pixel also decreases. Regardless of this decrease in the amount of incident light, the photoelectric conversion efficiency in the direction of depth of the photodiode must be improved in order to increase photoelectrons, i.e., a deep photodiode must be formed. Forming a deep photodiode means extending the depletion layer of the photodiode deep into the substrate. In this way, electrons that were generated by photoelectric conversion in the prior art but that flowed in the direction of the substrate and therefore were not accumulated in the photodiode portion and did not contribute to the sensitivity of the photodiode can be efficiently accumulated in the photodiode.
Based on this consideration, the assignees of the present invention have previously proposed in JP, P2001-7309A a method in which the impurity concentration of only the p-type well layer below the photodiode is made lower than the impurity concentration of the p-type well of other portions, thereby extending the depletion layer of the photodiode more deeply into the substrate and improving sensitivity to light, and moreover, reducing capacitive coupling with the substrate to improve detection sensitivity.
However, a problem remains that was not solved by the adoption of these measures. This problem involves the occurrence of crosstalk between pixels and the substantial decrease in resolution due to the movement of electrons between neighboring photodiodes (i.e., unit cells). In other words, the diffusion of photoelectrons in pixels in which light is generated results in the leakage of electrons into pixels in which there should be no incident light to begin with, and noise is thus generated that gives the appearance of signal input even in these pixels in which there is no incident light. Contrivances such as shield films are unable to suppress this crosstalk component because it originates from the generation of photoelectrons in relatively deep portions of the substrate. Light that results in photoelectrons in relatively deep portions of the substrate is light of wavelengths ranging from red to near-infrared. This is because, due to the dependence on wavelength of the absorption coefficient of silicon (Si) which is used as the semiconductor substrate, absorption length is greater for light of the red or near-infrared wavelength region, i.e., this light penetrates deep into the substrate. Photoelectrons that occur deep in the substrate due to the absorption of this light diffuse in the p-type substrate, and photoelectrons that diffuse in the horizontal direction in the sectional view shown in FIG. 2 or FIG. 3 may reach a neighboring pixel and thus become a source of crosstalk.
In contrast, blue and green light, which is visible light of short wavelengths, is substantially absorbed within portions of the n-type photoelectric conversion region and depletion layer of the photodiode that spread in the direction of the substrate, and this light therefore does not penetrate deep into the substrate. In other words, the crosstalk component which passes through the substrate is small for short wavelengths.
If the p-type well is formed to too great a depth, sensitivity with respect to near-infrared light increases and the device is no longer amenable for use as a solid-state imaging device for application to visible light. Infrared (IR) cutoff filters are often used to suppress sensitivity to near-infrared light, but if sensitivity to near-infrared light is too great, sensitivity to light that passes through the filter cannot be ignored regardless of the use of the IR cutoff filter.
Thus, simply increasing the depth of the p-type well to raise sensitivity increases the unwanted sensitivity to near-infrared light, and further, electrons that occur in the deeper portions of the p-type well diffuse horizontally and become a source of crosstalk to neighboring pixels.
In conclusion, a photodiode structure that provides both high sensitivity to visible light and low crosstalk has not yet been proposed.
It is an object of the present invention to provide a solid-state imaging device that can raise detection sensitivity, raise output conversion efficiency, raise photoelectric conversion efficiency, extend the depletion layer, maintain the isolation characteristics between pixels, and reduce the occurrence of crosstalk.
The object of the present invention is realized by a solid-state imaging device which includes: a substrate layer which is composed of a semiconductor of a first conductive type; a semiconductor layer of the first conductive type which is provided on the substrate layer; and a photoelectric conversion region of a second conductive type, which is the opposite conductive type of the first conductive type, provided on the semiconductor layer; wherein the impurity concentration of the substrate layer is higher than the impurity concentration of the semiconductor layer.
A low-impurity concentration semiconductor layer of the first conductive type having a lower impurity concentration than the semiconductor layer may be provided between the substrate layer and the semiconductor layer. In a case in which the low-impurity concentration semiconductor layer is provided, the impurity concentration of the low-impurity concentration semiconductor layer is preferably lower than the impurity concentration of the photoelectric conversion region.
The solid-state imaging device of the present invention may further be provided with a peripheral circuit semiconductor layer of the first conductive type that has a higher impurity concentration than the semiconductor layer and that is formed on the semiconductor layer at least below transistors in pixels. In addition, from the standpoint of suppressing sensitivity to near-infrared light, the distance from the surface of the main plane of the semiconductor to the surface of the semiconductor layer on the substrate layer side is preferably a minimum of 2 xcexcm and a maximum of 10 xcexcm.
In the present invention, at least one of the semiconductor layer and the low-impurity density semiconductor layer is preferably formed by epitaxial growth. Further, the depth profile of the impurity in the semiconductor layer is preferably a retrograde profile.
By adopting the above-described structure, the present invention can realize a photodiode that is capable of high sensitivity, can cause the spectral characteristic to approach the spectral characteristic of visible light, and further, can provide a solid-state imaging device that suppresses crosstalk between pixels.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.